Liquid-crystal display elements which have been developed for use in transmission type liquid-crystal displays include a twisted nematic liquid crystal (TN-LCD), super-twisted nematic liquid crystal (STN-LCD), twisted nematic liquid crystal (TFT-TN-LCD) utilizing a thin-film transistor (TFT), vertical-alignment liquid crystal (VA-LCD), and in-plane switching liquid crystal (IPS-LCD).
These transmission type liquid-crystal displays comprise a liquid-crystal cell formed by sandwiching a liquid-crystal element between glass substrates and polarizing plates disposed respectively on the upper and lower sides of the cell. The polarizing plates have been disposed so that the absorption axes thereof are perpendicular to each other in order that the display, when viewed from a front direction for the polarizing plates, might have a light transmittance of almost 0% to thereby make the orientation of the liquid crystal visually recognizable. However, there has been the following problem. When this display is viewed from directions which are oblique to the front direction and make an azimuth of 45° with the directions of the absorption axes of the polarizing plates, then the polarized light which has passed through the entrance-side polarizing plate is not sufficiently absorbed by the emission-side polarizing plate and light leakage hence occurs, resulting in a narrowed viewing angle.
In recent years, the desire for an improvement in image quality is growing simultaneously with the enlargement of the market for transmission type liquid-crystal displays. In particular, there is a desire for an improvement in image quality and widening of a viewing angle by compensating for the geometrical axial shifting of polarizing plates because polarizing plates are employed in all transmission type liquid-crystal displays.
The relationship between a polarizing plate and/or optical compensation film and viewing angle is shown in FIG. 1. In FIG. 1, (a) indicates a direction normal to the plane of the polarizing plate and/or optical compensation film; (b) indicates the direction of the slow axis of the optical compensation film which has been oriented by stretching; (c) indicates an elevation angle with respect to the plane of the polarizing plate and/or optical compensation film; and (d) indicates an azimuth on the plane of the polarizing plate and/or optical compensation film.
The geometrical axial shifting of polarizing plates is a phenomenon in which when a liquid-crystal display comprising a liquid-crystal cell and a pair of polarizing plates disposed respectively on the upper and lower sides of the cell so that their absorption axes are perpendicular to each other is viewed from a direction which is oblique to the optical axis for the polarizing plates and is different from the directions of the absorption axes of the polarizing plates, then light leakage occurs and this results in a narrowed viewing angle. For example, when the liquid-crystal display is viewed from directions forming an azimuth of 45° with the absorption axes of the polarizing plates and forming various elevation angles with the direction normal to the polarizing plates, then the angle between the absorption axes of the polarizing plates disposed respectively on the upper and lower sides of the liquid-crystal cell is 90° or larger and, hence, light leakage occurs, resulting in a narrowed viewing angle.
A polarizing plate compensating for such geometrical axial shifting of polarizing plates which is observed in viewing from oblique directions was proposed. This polarizing plate employs a sealing film showing retardation as a transparent protective film for the polarizer (see, for example, JP-A-04-305602 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)). It has also been proposed to stack a polarizing plate and a retardation film exhibiting positive birefringence (see, for example, JP-A-2004-157523).
Positive birefringence means the property of having such refractive-index anisotropy that a film which has been stretched to cause molecular chains of the polymer as a component of the film to undergo molecular orientation has an increased refractive index in the direction parallel to the stretching direction. On the other hand, negative birefringence means the property of having such refractive-index anisotropy that a film which has been stretched to cause molecular chains of the polymer as a component of the film to undergo molecular orientation has a reduced refractive index in the direction parallel to the stretching direction and simultaneously has an increased refractive index in the direction perpendicular to the stretching direction.
However, the proposal in JP-A-04-305602 has had drawbacks, for example, that the sealing film used as a transparent protective film for a polarizer is difficult to adhere to the polarizer in production. The proposal in JP-A-2004-157523 has had drawbacks, for example, that it necessitates a stretching technique which is not for producing a mere uniaxially stretched film but for controlling orientation in film in-plane directions and the film thickness direction as used in the Examples and it is difficult to stably orient in in-plane directions of the film and the film thickness direction, and that the stretching is costly. Furthermore, there has been a drawback that when a liquid-crystalline polymer is used, it is difficult to evenly orient it and to evenly impart the property of performing optical compensation.